The Fedora developers have decided that Fedora 16 will not use Btrfs by default. The announcement was made by Josef Bacik, Senior Software Engineer at Red Hat. Instead the switch to Btrfs filesystem has now been postponed to Fedora 17.

SElinux: I think it was disappointing to a lot of people ("hey look at this cool new security feature? too bad we can't use it because our software breaks") but if you ran into an issue with it, it was easy to switch off.

In addition to the video drivers, I'd add pulseaudio and kde4. Both of these initially led to bad user experiences for a lot of people.

However I think the difference between those and making btrfs default is that no one wants to risk losing their data. For example having my music stutter when the system load gets high is annoying but trivial in comparison to finding out that a ten page report I was working on was corrupted because of some obscure FS bug.